1. Field of the Invention
The present invention relates generally to web printing presses and more particularly to a method and apparatus for determining the presence of slipping of a driven nip roller in a web printing press.
2. Background Information
Web printing presses print a continuous web of material, such as paper. The web travels through nips formed by opposing nip rollers. The web is moved on its way by driven nip rollers which are driven by respective nip roller drivers. Slipping of the driven nip rollers relative to the web can adversely affect printing press performance. With non-slipping driven nips attenuation of tension disturbances is reduced.
Tension in the web must be maintained within a desired range in order to achieve smooth operation of the printing press. At the same time, the velocity of the web, and hence the rotational speed of non-slip nip rollers, must be held relatively constant to achieve good print product quality. Tension in a web span between two nips can be adjusted by controlling the speed of the nip roller driver in the downstream nip.
The press operator has had no simple way to determine whether a driven nip roller is slipping or not. Even if the press operator determines that a driven nip roller is slipping, reestablishing a non-slip condition of the nip roller can be difficult and time-consuming because, among other things, the operator may not be able to easily determine whether the nip roller is slipping in a positive direction, i.e., where the tangential speed of the nip roller is greater than the speed of the web, or whether the nip roller is slipping in a negative direction, i.e., where the tangential speed of the nip roller is less than the speed of the web. Additionally, a driven nip roller may be non-slipping at one nominal, or command, press speed and transition to slipping at another nominal press speed. This transition may not degrade the operation of the press enough to prompt operator intervention, yet the slipping may reduce attenuation of tension disturbances to unacceptable levels.
Japanese Patent Document No. JP11286358 purports to describe a slip restraint control method for a roller conveying a band of metal. When slipping takes place, the speed of rotation of the motor driving the roller is changed to vary between the slipping range and the non-slipping range, while monitoring the current to the motor. An optimum motor speed is determined based on the resulting data.
What is needed is a way to automatically determine and indicate a slipping nip roller condition.
An object of the present invention is to provide a method and apparatus for determining the presence of slipping of a driven nip roller in a web printing press.
The present invention provides a method for determining the presence of slipping of a driven nip roller of a nip relative to a web passing through the nip in a web printing press, the method comprising: causing a plurality of changes in a speed of a driver driving the nip roller; monitoring a respective change in the difference in tension in the web upstream and downstream of the nip upon each respective change of the speed of the driver so as to determine a linearity and a slope of a relationship between the difference in tension and the speed of the driver; and determining the presence of slipping when the difference in tension is non-linear with respect to the speed of the driver or the slope is substantially less than an expected slope.
The presence of non-slipping of the driven nip roller is determined when the difference in tension is linear with respect to the speed of the driver and the slope is near the expected slope.
The expected slope may be a function of an estimated or measured modulus of a material of the web.
The plurality of changes in speed may include an increase in speed and/or a decrease in speed.
An operator indication of the presence, as well as the direction, of the slipping may be provided.
In the presence of slipping, the tension set point of a web tension control system for the web upstream of the nip may be increased, by the press operator, for example, so as to achieve a non-slipping of the driven nip roller. Depending on the operating mode of the press, the velocity ratio of a driver control system of the driver may be increased so as to achieve a non-slipping of the driven nip roller.
Alternatively, in the presence of slipping the force urging together the nip rollers of the nip may be increased so as to achieve a non-slipping of the driven nip roller.
The present invention may further comprise: determining a first tension range for the web upstream of the nip, the nip roller being non-slipping in the first tension range; and controlling the driver so as to maintain the tension in the web upstream of the nip in the first tension range. In an embodiment of the present invention, the tension may be maintained at a value near the middle of the first tension range.
The present invention may further comprise: determining a speed range of the driver at a predetermined difference in tension, the nip roller being non-slipping when the speed of the driver is in the speed range; and controlling the driver so as to maintain the speed of the driver in the speed range. In an embodiment of the method according to the present invention, the speed may be maintained at a value near the middle of the speed range.
The printing press may include a second nip upstream of the nip and a third nip downstream of the nip, with the second nip including a second driven nip roller and the third nip including a third driven nip roller. The method according to the present invention may further comprise controlling the second driven and/or the third driven nip roller so as to establish a desired value of the difference in tension. The driven nip roller is preferably non-slipping when the difference in tension is at the desired value.
The method according to the present invention may further comprise: determining a speed range of the driver at a predetermined difference in tension, the driven nip roller being non-slipping when the speed of the driver is in the speed range; and controlling the driver so as to maintain the speed of the driver in the speed range. In an embodiment of the method according to the present invention, the speed of the driver at a value near a middle of the speed range.
The present invention also provides a method for determining the presence of slipping of a driven nip roller of a nip relative to a web passing through the nip in a web printing press, the method comprising: causing a plurality of changes in a speed of a driver driving the nip roller; monitoring a respective change in a tension in the web upstream of the nip upon each respective change of the speed of the driver so as to determine a linearity and a slope of a relationship between the tension and the speed of the driver; and determining the presence of slipping when the tension is non-linear with respect to the speed of the driver or the slope is substantially less than an expected slope.
Additionally, the present invention provides a method for determining the presence of slipping of a driven nip roller of a nip relative to a web passing through the nip in a web printing press, the method comprising: causing a plurality of changes in a speed of a driver driving the nip roller; monitoring a respective change in a torque of the driver upon each respective change of the speed of the driver so as to determine a linearity and a slope of a relationship between the torque and the speed of the driver; and determining the presence of slipping when the torque is non-linear with respect to the speed of the driver or the slope is substantially less than an expected slope.
The present invention also provides an apparatus for determining the presence of slipping of a driven nip roller of a nip relative to a web passing through the nip in a web printing press. A first tension sensor is disposed upstream of the nip and having a first tension output being a function of a tension in the web upstream of the nip. A second tension sensor is disposed downstream of the nip and having a second tension output being a function of a tension in the web downstream of the nip. A driver is provided for driving the driven nip roller. A processor is provided for causing a plurality of changes in a speed of the driver. The processor receives the first and second tension outputs and determines for each of the changes in speed a respective difference in tension in the web upstream and downstream of the nip based on the first and second tension outputs so as to determine a linearity and a slope of a relationship between the difference in tension and the speed of the driver. The processor determines that slipping is present when the difference in tension is non-linear with respect to the speed of the driver or the slope is substantially less than an expected slope.
An indication device may be included for providing an operator indication of the presence, as well as the direction, of the slipping based on an output from the processor.
The processor may be included in a press tension control system.
The processor may be designed to carry out the various embodiments and aspects of the method according to the present invention described above. For example, the processor may implement one or more algorithms for performing the method according to the present invention.